Synthetic mold composition and method of making same



M UPI 194 a 7 s rum. 758.817

This invention relates to sand molds of dimcultly' oxidizable metals ofhigh melting-point, as grey iron, steel, bronze; refrac torynon-ferrousmetals and alloys thereof but more especiallyto ferrous metals. g Amongthe principal objects of the "invention are the provisions of a novelsynthetic moldingf medium, composed principally of sand, which, whentempered with-water and formed into molds, in the manner customary whenemploying ordinary molding sand,*'imparts to such molds highly desirablecharacteristics and more especi-' ally the capability of'producingcastings oi the aforesaid metals from which'the molding sand can bereadily stripped without resorting to ex pensive prolonged tumbling,wire-brushing, sand- 5 blasting and like finishing operationa' wherebyremarkably smooth and pattern-true castings, e's-" sentially free, fromburnt-'on-sand grains, canthe inventionf are:

obtained. Otherv objects of hereinafter set forth. Y a I HGIEtOfOIQ B-SIamfwell aware, special facing the'castingin" green 1 sands have been:employedi foundries, for mg the m1dsu$eina$ting metals, "suchaforesaid, a which were I composed principally of either naturally: orartificially bonded molding" sand vwith. which was intimatelyincorporated a" modicum of high melting-point pitch, commonly g in aratio of 90 of sand-to 1 of pitch, in an-effort to minimizetheflso-called burn-on of the sand therefrom. p Y investigations velopthe bonded particl a: themolding sand in the facing'layer: Such actiontends to objectionably waterproof eithe'r'the' naturally or 'art'incial-3 1y bonded sand'partieles', so enveloped, to such an extent thateventually the "entire molding sand heap can no longefabsorbsuiflcientwater to bfe capableof being properly tempered, so tospeak,-when it was attempted to form fresh t have led to the'disco 'veryof a special facing sand, containing pitch crpitchlike substances as anactive soot p'roduc'ing ingredientthereof; which are'entirelyiiree'fromthe H objectionable feature ordinarily impartedthereto by pitch. thesame beingprepared inaccordance with the procedure set forth-in thefollowing detailed-description 'thereofi proceed as-follows: p I

200jparts by-weight of sharp or unbon'dedsilica sandof the desired grainsizes are intimateiyad mixedin a suitable mixer equipped with paddies,with 10 parts; 'by weight, of powdered "deal .In carryingoutmy'invention is it preifeifable tar'pitch, preferablynormallysolidpitch ofa melting point of=400 to 500 F.,*th'e mixture beingheated, preferably by the application ofdirect heat; until the pitch isthoroughly melted and the same being continually stirred in order my thoroughly distribute the melted pitch so that' the sand 'grainswill becomeenveloped by"a' flayer of I the hot "stickyplt'ch or other hydrocarbons,Such grains to the casting due to the normaltendency' of themoltenmetalto'fusethe bonding'material-"' of the molding sand both to themetal and to the sand grains .in the. mold metal' interface. Suchfacing'sands were applied next-to the patt'ern 'as' a facing layer ofs'ubstantialthickness, usuallyfrom about %""toan inch or more, and when'the molten metal entered the'moldca'vity, "faced with such special"sands,"the volatile hydrocarbons-inf the pitch component thereof wererapidly vola 1 tilized by the hot 'met'al and the evolved-vapors]ignited to produce a 's'oot'y flame'due to the limiited supply ofunconsunied oxygen present in the 1 mold. As a resultof such'incompletecombustion; f the sand particles at' the mold -m'etal interface'j becamecoated with soot which acts to'se arate the sand-nuclei of suchcoatedparticlesfrorn'the hot metal and thus materially aids in'thestripping ofthe'sand layer at such interface front the finished casting. v p

Among the disadvantages attendant upon the use of pitch'in suchfacin'gsands was -the tendency of the sametormelt at a relatively low temperaupon so melting to run freely around and enture varying, from: about 300400 "and as -.gilsonite,' el-aterite, asphalt, petroleumf's'tillbottomresidues, capable' of forming voluminous quantities of so'ot' onburningin a limited supply or air which may, 'ifde'sired,'be'satisiactorilyfisub f stituted for such pitch;-- Wherrthesandgrains havebec'ome thoroughly'coated; 20' part'siofpow deredcarbon,-- essentially'free from volatiles, de-' sirably 72 hr.metallurgical -coke,''p'etrole u'm coke or retort hen-age addedandth'ereupon" the heating isdiscontinued" while the'jstirring is continued until the previously co'ated sticky f'sand particles are inturnhomogeneouslycoated with}; layer of'the high refractorycaljbonpowderg The agitation ofthe mass is (:ontin ed uhtilthejtemperature thereof normally drops'iuntilitbecomes cold brat-least dropsbelow the melting p oint, of mm ing', the pitch component of the r H Iifle'sfand anchors the coke carbd layer fast tofthe silica grains.Thej'r' esultant' mass is dumped from; the mixer,- ;screened" to "removeany; lumps afnd'fthe ound, rescreen'edf separated lumps are their g andadded to tlieother finished material.

In order to prepare the molding medium for use r c i droe on e ren nced;

SS, again. SOlid in making green sand molds from such finished binders,until the desired green bond and green shear strengths are developed inthe molding medium, dependent upon the particular class of work forwhich it is intended to be used.

Another important advantage such novel molding medium, is the fact thatit lends itself to the production of baked cores for use in casting theaforesaid metals in green sand molds, it being merely necessary to formcores of the desired size and shape from such medium after suitablybonding the same with appropriate bonding agents" and binders and thento bake such cores, in th manner now customary, to impart thereto therequisite hot strength.

In lieu of employing hydrocarbons, such as aforesaid, as the stickingagent, appropriate amounts of various carbohydrates, as molasses,

dextrine. sulphite waste liquors or sticky residues obtained. asby-products from alcohol distillation, may be substituted therefor, butin such an event the mass in the mixer must be heated sufficiently tochar the carbohydrate and render it insoluble and such charring actionfunctions of itself to anchor the subsequently added carbon fast to thesilica grains substantially as effectively as do the hydrocarbons asaforesaid.

The successive heating-and-cooling pre-treatment of the hydrocarbon andsand mix, on the one hand, or the carbohydrate and sand mix, on theother hand. not only fixes the hydro-carbon or the carbohydrate so thatit will not flow during the casting operation but drives off such of thevolatiles of these compounds as would tendto cause mapping, gas marks,or rounded edges on the casting surfaces and insures against anypossibility of the explosion of the mold when the same is broken up suchas sometimes occurs when external air comes in contact with volatilesevolved from the priorpitch-treated facing sands. The coating of thesticky sand particles with the powdered carbon which desirably is fromabout 200 to 325 mesh or even finer, insures of the deposit of aninsulating layer of refractory material on the sand particles that isfar moreeifective in preventing the burn-on of sand to the casting thanit is'possible' to accomplish when employing ordinary pitch-treatedfacing sand.

Castings produced from molds prepared fro my improved molding medium canbe readily stripped from the sand layer at the mold-metal interface andremarkably smooth pattern-true castings obtained without necessitatingprolonged and expensive mechanical cleaning operations thereon. So alsocan the cores prepared from such molding medium, as aforesaid, bereadily stripped from castings formed therewith and such corescontribute to the production of remarkably pattern-true holes in suchcastings whose surfaces are unusually smooth.

The amount of pitch to be incorporated with any given amount of sharpsilica sand that is employed will vary directly with the grain size ofsuch sand or like refractory material, since the finer the grain size,the greater will be the surface presented and therefore the greater thequantity of pitch that will be required. Desirably between 3% to 17% ofthe pitch by weight of the sand-pitch mixture is employed.

Various modifications of the procedure and proportions above specifiedmay be resorted to when making my improved molding composition withoutdeparting from the spirit of my invention as embraced within the scopeof the appended claims.

Having thus described my invention I claim:

1.- An intermediate composition for use in making'a mold compositionconsisting essentially of sharp silica sand grains externally coatedwith pitch having a melting point of at least about 400 F.-and in whichare embedded particles of refractory powdered carbon, the amount of suchrefractory carbon being suilicient to prevent burn-on of the silica sandgrains to a metal casting during the forming thereof ina mold formedwith such intermediate composition.

-2. A mold, composition for metal-castingpurposes consisting essentiallyof sharp silica sand grains extemaily coated with pitch having a meltingpoint between about 400 F. and 500 F. having a layer of refractorypowdered carbon de-.- posited thereon and a modicum. of a clay-like bondincorporated with said mold composition, the particles of saidbond beingout of direct contact with said unbonded sand grains and the amount ofsuch refractory carbon present in the mold composition being sufllcientto prevent the burn-on of the silica sand grains to a metal castingduring the forming thereof in a mold formed with such mold composition.

3. A mold composition for metal-casting purposes consisting essentiallyof sharp silica sand grains which are individually externally coatedwith high melting-point pitch that is essentially free from volatilesand in which pitch coatings are embedded particles'of refractorypowdered carbon, the said mold composition containing a modicum of aclay-like bond, the particles of which are out of direct contact withsaid sharp silica grains and theamount of such refractory carbon presentin the mold composition being suificient to prevent the burn-on of thesilica sand grains to a metal casting during the forming thereof in amold formed with such mold composition.

4. A mold composition for metal-casting purposes, consisting essentiallyof sharp silica sand grains the particles of which are individuallycoated with a film of pitch having externally embedded therein fineparticles of refractory carbon, said mold composition containing amodicum of a clay-like bond the particles of which are out of contactwith the sharp silica sand grains and theamount of such refractorycarbon present in the mold composition being sufficient to prevent theburn-on of the silica sand grains to a metal casting during the formingthereof in a mold formed with such mold composition.

5. In the method of making a mold composition .for metal-castingpurposes, the steps which comprise intimately admixing particles ofsharp silica sand of selected grain sizes with pitch, heating themixture with constant stirring to a suflicient temperature to melt thepitch andeffect the substantial coating of the sand grains, thenadmixing powdered refractory carbon with the resultant mass while thepitch therein is still in a tacky condition so as to effect theanchoring of the carbon powder to the coated sand particles, the amountof such refractory carbon employed being sufficient to preventtheburn-on of the silica sand grains to a metal casting during the formingthereof in a mold formed with such mold composition so obtained.

6. In the method of making a mold composition for metal-castingpurposes, the steps which comprise effecting the individual coating ofsharp silica sand grains with a fllm of hot pitchrthen while the filmsare still hot and in a tacky condition dusting into the mass finelypowdered refractory carbon while continually stirring the mass andthereby efiecting the anchoring of the particles of powdered carbon insaid films of pitch, the amount of such refractory carbon employed beingsumcient to prevent the burn-on of the silica sand grains to a metalcasting during the forming thereof in a mold formed with such moldcomposition so obtained.

BURGESS P. WALLACE. u

. Q nnmamcns crrm The following references are of record in the file ofthis patent:

Germany Oct.'12, 1931

